This invention relates to a method of, and apparatus for use in, magnetic resonance imaging; and more particularly, to contrast agent enhanced magnetic resonance angiography for examining, detecting, diagnosing, and treating arterial diseases and injuries, including defining anatomic features relevant to performing aorta and aortic surgery for aneurysmal disease.
Arterial diseases and injuries are common and often have severe consequences including death. Imaging arteries serves to detect and characterize arterial disease before these consequences occur as well as defining anatomic features to assist in performing surgery for aneurysmal disease.
A conventional method of arterial imaging includes inserting a catheter into the artery of interest (the artery under study) and injecting radiographic contrast, for example, an iodinated contrast, while taking radiographs of the artery. Radiographs are commonly referred to as X-rays. In this technique, the contrast remains in the arteries for a few seconds during which the arteries appear distinct from both the veins and background tissue in the radiographs.
Although a catheter-based contrast arteriography technique generally provides high quality arterial images, there is a risk of arterial injury or damage by the catheter and its insertion. There may be thrombosis, dissection, embolization, perforation or other injury to the artery itself. Furthermore, such a technique may result in a stroke, loss of a limb, infarction or other injury to the tissue supplied by the artery. In addition, hemorrhage at the catheter insertion or perforation sites may require blood transfusions. Moreover, kidney failure and brain injury may result from the toxic effects of the X-ray contrast.
More recent techniques of arterial imaging are based upon detecting the motion of the blood within the arteries and/or veins.
These techniques involve employing magnetic resonance imaging (MRI) to image moving blood distinct from stationary background tissues. (See, e.g., Potchen, et al., eds., xe2x80x9cMagnetic Resonance Angiography/Concepts and Applicationsxe2x80x9d, Mosby, St. Louis, 1993; the text of which is incorporated herein by reference). Such techniques do not necessitate catheter insertion into the artery. These techniques are commonly known as 2D time-of-flight, 3D time-of-flight, MOTSA, magnitude contrast, phase contrast, and spin echo black blood imaging.
With pre-saturation pulses it is possible to primarily image blood flowing in one direction. Since arteries and veins generally flow in opposite directions, these pre-saturation pulses allow preferential visualization of the arteries or the veins. Because these techniques depend upon blood motion, the images are degraded in patients who have arterial diseases which decrease or disturb normal blood flow. Such types of arterial diseases that decrease or disturb normal blood flow include aneurysms, arterial stenoses, arterial occlusions, low cardiac output and others. The resulting lack of normal blood flow is particularly problematic because it is those patients with disturbed blood flow in whom it is most important to acquire good quality arterial images.
A related MRI technique relies on differences in the proton relaxation properties between blood and background tissues. (See, e.g., Marchal, et al., in Potchen, et al., eds., supra, pp. 305-322). This technique does not depend upon steady blood in-flow. Instead, this MRI technique involves directly imaging the arteries after administering a paramagnetic contrast agent. Here, after administering the contrast agent, it is possible to image arteries directly based upon the blood relaxation properties. This technique overcomes many of the flow related problems associated with MRI techniques which depend upon blood motion.
Several experts have performed magnetic resonance arterial imaging using intravenous injection of gadolinium chelates (paramagnetic contrast agents). These experts have reported their results and conclusions. In short, these results have been disappointing and, as a result, the use of gadolinium for imaging arteries has not been adopted or embraced as a viable arterial imaging technique. The images using this technique are difficult to interpret because the gadolinium tends to enhance both the arteries and the veins. Since the arteries and veins are closely intertwined, it is extremely difficult to adequately evaluate the arteries when the veins are visible. Further, the difficulty in interpretation is exacerbated as a result of contrast leakage into the background tissues.
However, MRI has evolved over the past decade to become an accepted technique to image the abdominal aorta and abdominal aortic aneurysms. Advances in magnetic resonance imaging for vascular imaging, known as magnetic resonance angiography, have enabled the additional evaluation of aortic branch vessels. However, limitations in magnetic resonance angiography imaging of the slow, swirling flow within aneurysms, turbulent flow in stenoses, and tortuous iliac arteries have limited the usefulness of these general studies in providing detailed information necessary for preoperative planning. In spite of these limitations, recent developments in gadolinium-enhanced magnetic resonance angiography have overcome several of the imaging problems. (See, e.g., Debatin et al., xe2x80x9cRenal magnetic resonance angiography in the preoperative detection of supernumerary renal arteries in potential kidney donorsxe2x80x9d, Invest. Radiol. 1993;28:882-889; Prince et al., xe2x80x9cDynamic gadolinium-enhanced three-dimensional abdominal MR arteriographyxe2x80x9d, JMRI 1993;3:877-881; and Prince, xe2x80x9cGadolinium-Enhanced MR Aortographyxe2x80x9d, Radiology 1994;191(1):155-64).
There exists a need for an improved method of magnetic resonance angiography which provides an image of the arteries distinct from the veins and which overcomes the limitations of other techniques. Further, there exists a need for an apparatus which facilitates providing an image of the arteries distinct from the veins and which may be implemented in overcoming the limitations of other techniques.
Moreover, these exists a need for contrast (e.g., gadolinium) enhanced magnetic resonance angiography of abdominal aortic aneurysms to provide essential and accurate anatomic information for aortic reconstructive surgery devoid of contrast-related renal toxicity or catheterization-related complications attending conventional arteriography.
In one aspect, the present invention is a method of imaging an aorta and aortic aneurysm of a patient using magnetic resonance imaging. The method includes performing a first imaging sequence to identify the location of the aneurysm and performing a second imaging sequence to image the aorta and extent of the aortic aneurysm. The second imaging sequence includes collecting image data and administering magnetic resonance contrast agent to the patient prior to and/or while collecting image data, by intravenous infusion, at a rate of infusion sufficient to provide a substantially elevated concentration of the contrast agent in the artery during collection of image data representative of a center of k-space.
The first imaging sequence may be a sagittal T1 weighted sequence. The second imaging sequence may be a plurality of images constructed from a dynamic 3D volume. The plurality of images of the second imaging sequence may include a plurality of coronal, sagittal or oblique projections.
The method may further include the step of performing at least a third imaging sequence for imaging the size of the aortic aneurysm. The third imaging sequence may be performed after performing the second imaging sequence. In a preferred embodiment, the third imaging sequence is a plurality of sagittal or axial 2D time-of-flight images and further includes collecting imaging data while the patient suspends respiration.
In another preferred embodiment, the method includes the step of performing a fourth imaging sequence for imaging the size of the aortic aneurysm wherein the third and fourth imaging sequences are a plurality of sagittal and axial 2D time-of-flight images.
In yet another preferred embodiment, the invention includes performing a fifth imaging sequence for imaging right renal arteries. The fifth imaging sequence may include collecting data representative of phase contrast images.
In another aspect, the invention is a method of imaging portions of the aorta and its major branches in a patient using magnetic resonance imaging. The method includes performing a first imaging sequence to identify the location of the aorta and performing a second imaging sequence to image a lumen of the aorta. The second imaging sequence includes collecting image data representative of the center of k-space while the patient suspends respiration. The second imaging sequence further includes administering magnetic resonance contrast agent to the patient, by intravenous infusion, at a rate of infusion sufficient to provide a substantially elevated concentration of the contrast agent in the artery during collection of image data representative of a center of k-space.
In a preferred embodiment, the first imaging sequence is a sagittal T1 weighted sequence. In another preferred embodiment, the second imaging sequence is a 3D gradient echo volume.
In another preferred embodiment, the method includes a third imaging sequence, following the step of administering magnetic resonance contrast agent, for collecting 3D phase contrast images.
In yet another aspect, the present invention is a method of imaging aorta or renal arteries of a patient using magnetic resonance imaging. The method includes performing a first imaging sequence to identify the location of the aorta and aorta branch vessels and performing a second imaging sequence to image a lumen of the aorta. The second imaging sequence includes collecting image data and administering magnetic resonance contrast agent to the patient prior to or while collecting image data, by intravenous infusion, at a rate of infusion sufficient to provide a substantially elevated concentration of the contrast agent in the artery during collection of image data representative of a center of k-space.
The step of performing the second imaging sequence may include collecting at least a portion of the image data while the patient suspends respiration. In a preferred embodiment, the step of performing the second imaging sequence includes collecting at least a portion of the image data corresponding to the center of k-space while the patient suspends respiration.
The present invention overcomes the limitations of other techniques by injecting magnetic resonance contrast agents at a sufficient rate, at a selected time relative to the collection of image data, and for an appropriate duration in such a manner that the contrast level in the arteries is higher than that in surrounding veins and background tissue during collection of image data. The injection may be intravenously in a vein remote from the artery of interest. Intravenous injection eliminates the risks associated with arterial catheterization. In the present invention, the high level of arterial contrast permits directly imaging the arterial lumen, analogous to conventional arteriography. Moreover, using a magnetic resonance pulse sequence which is not as sensitive to motion and by relying on image contrast related to differences in T1 relaxation rather than the in-flow effect, a reduction in the flow artifacts associated with phase contrast or magnitude contrast (time-of-flight) magnetic resonance angiography is observed.
In short, the present invention is, in comparison or relative to other techniques, a method of magnetic resonance angiography which combines several of the advantages of catheter-based contrast arteriography with the advantages of magnetic resonance imaging while substantially eliminating the disadvantages of each.